1. Field of the Invention
The present invention is directed to a fixing unit and an image forming apparatus equipped with the fixing unit.
2. Description of the Prior Art
In general, an image forming apparatus such as a printer, a copying machine, a fax machine or the like which uses electronic photography is provided with a fixing apparatus. The fixing apparatus is provided for heating a recording medium such as paper or the like which carries a toner image formed from toner in an unfixed state as well as for applying pressure thereto to fix the toner image onto the recording medium (see JP-A No. 2004-262976).
Such a fixing unit includes a pair of cylindrical rollers (rotating bodies) which are rotated with being pressed to each other, and a heat source which heats at least one of these rollers. By passing the recording medium through the nip formed by the pair of rollers which are pressed to ea ch other, and applying heat and pressure to the recording medium, the toner image formed on the recording medium is fixed onto the recording medium.
Recently, according to the increased printing speed, the heater tends to consume a greater amount of electricity for the sake of, e.g., shortening a warm-up time. The heater with increased electricity consumption has a small resistance, which involves a problem in that a surge current may flow therethrough to thereby generate what is called a “flicker” phenomenon at the time when the connection to an electric power source is switched from an off-state to an on-state. Particularly, this problem becomes conspicuous at the time when the heater remains at a lower temperature, because the resistance of the heater is also kept small at this time.
In order to solve this problem, there is known a method of increasing the resistance value of a heater-containing circuit by connecting a heater and a resistor to a power source in series when the connection of the heater (first heater) with high electricity consumption to the power source is switched from an off-state to an on-state.
A heater (second heater) is used as the resistor in the Japanese publication cited above. This makes it possible to avoid any occurrence of flicker and, in addition, to efficiently employ the heat energy generated by the heater in heating the afore-mentioned rotating bodies or other components.
However, even with the combination of resistance values of the first heater and the second heater described above, there has still existed such an instance that the heating value of the first heater becomes too small when the first heater and the second heater are energized in series, thus making it impossible to avoid the occurrence of flicker.